Because of their components (optical fibers, amplifiers, etc.), optical transmission lines suffer power losses that degrade transmission performance and can in some circumstances lead to processing errors and even to loss of data. While the line is being installed, the nominal power losses in the fiber can be measured and devices for statically compensating them can be adjusted accordingly. However, optical power losses vary with time. The variations in the losses are usually related to aging of components and/or to maintenance or repair work.
To track changing power losses, it has been proposed to measure the optical output power of the primary optical signals at the exit end of the optical fibers periodically. This type of measurement cannot distinguish a loss induced by an optical fiber from a loss induced by an optical amplifier feeding the fiber with primary signals. US patent application 2001/0050807 describes a method of determining the losses in an optical transmission line including a fiber used both for transmission of an optical signal and for amplification of the signal by the Raman effect. This kind of amplification uses an optical pump source. The above prior art document proposes switching the pump source to compare the power of the signal received with and without pumping to deduce the losses in the fiber.
In lines provided with a supervisory channel, it is possible in theory to measure the input optical power of the primary signals to be transmitted and to transmit information representative of the measured power on the supervisory channel, using supervisory signals, so that the optical input power can be compared to the output power. However, this type of detection is complicated to implement, and most importantly requires processing time unsuited to a procedure for dynamically adjusting optical power.
The document JP 0827 1380 discloses a method which transmits a supervisory signal with information describing the supervisory signal in the form in which it exists prior to transmission. That method therefore has the drawback of necessitating transmission of information describing the supervisory signal.
It is desirable not only to measure dynamically the variations in the losses caused by a transmission fiber, but also to compensate dynamically the effect on the primary signals of those variations. Thus an object of the invention is to propose a method of dynamically measuring the variations in the losses and dynamically compensating their effect.